The present invention relates to a dopant diffusion barrier spike for use in III-V semiconductor structures.
The precise placement of p n and pi (p type-intrinsic) junctions in the active regions and blocking structures of optoelectronic devices is exceedingly important for meeting the increasingly stringent requirements placed on device performance, such as modulation bandwidth, output power, extinction ratio and uncooled operation.
One typical structure employed in optoelectronic devices is the p-i-n (PIN) structure. In a typical PIN structure, an intrinsic layer is disposed between a p-type layer and a n-type layer. Typically, heterojunctions are formed at the p-i and n-i interfaces. The intrinsic layer has a larger index of refraction than the p and n layers resulting in a natural waveguide. Furthermore, the energy band discontinuities in the conduction and valence bands in the PIN structure facilitate carrier confinement within the active layer. In short the PIN structure is well suited for variety of photoemitter and photodetector optoelectronic device applications.
One material that is often used in optoelectronics devices is indium phosphide (InP). In a PIN structure employing InP, a p-type InP (p-InP) layer is often fabricated by introducing zinc as a dopant. While zinc is a suitable dopant for forming a p-type layer, zinc can readily diffuse during the higher temperatures achieved in the growth of InP. Diffusion into the active (intrinsic layer in a PIN structure) layer can occur resulting in the undesired doping of the active layer. This can have deleterious effects. For example, in a laser, intra-band transitions can occur resulting in optical losses and reduced output power. Furthermore, the unintentional doping of the active layer can result in a shift in the emitted wavelength of an emitter. In applications where the PIN structure is used as an electroabsorptive modulator, zinc dopants in the active layer can change the absorption edge thereby degrading the extinction ratio.
In order to overcome the drawbacks of diffusion of zinc dopants into the active region, diffusion barriers or blocks have been employed. These diffusion barrier layers prevent zinc diffusion into the active layer, thereby overcoming the drawbacks discussed above. A very thin layer of silicon, for example, can be placed in the p-InP layer creating a good diffusion barrier layer. However, because silicon is a donor material, an undesired p-n junction can be formed in the p-InP layer. While preventing the diffusion of zinc dopants into the active layer, the formation of the p-n junction introduces an undesired parasitic capacitance which is particularly undesirable in high speed devices. The resulting p-n junction is parasitic and may result in carrier recombination. In a laser, for example, a higher drive current may be required. As would be readily appreciated by one of ordinary skill in the art, a higher drive current results in overheating of the device and, consequently, a wavelength shift of the emitted light.
Another technique used to prevent zinc dopant from diffusing into the active layer is the use of an undoped layer on InP, also referred to a set-back layer. While the InP set-back layer is somewhat successful in preventing the diffusion of zinc dopants into the active layer, the optimal thickness of the set-back layer is dependent upon the 3 doping level and thickness of the zinc doped cladding and contact layers. This creates undesired fabrication complexities. Moreover, the zinc doping profile is difficult to using a set-back layer. For. example, near the active region the p-type doping concentration and the p-InP layer can be low. In a laser, for example, this reduces carrier concentration resulting in reduced power and ultimately in reduced efficiency of the laser. To supply suitable output power, the drive current will be increased, resulting in heating problems to include heating induced wavelength shift.
Accordingly, what is needed is a technique for blocking the diffusion of zinc dopants from the p-InP layer of a optoelectronic device which overcomes the drawbacks of the techniques discussed above.
The present invention relates to an optoelectronic device having at least one dopant diffusion barrier spike disposed in a doped layer of the device. The dopant diffusion barrier spike material is chosen so that it will not create a pn junction within the doped layer, but will effectively prevent diffusion.
The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that in accordance with the standard practice in the semiconductor industry the various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.
FIG. 1 is a cross-sectional view of an exemplary embodiment of the present invention showing a diffusion barrier spike.
FIG. 2 is a cross-sectional view of an exemplary embodiment of the present invention showing multiple diffusion barrier spikes.
FIG. 3 is a cross-sectional view of a mesa structure having diffusion preventing barrier spikes within the p-type region as well as between the p-type region and the blocking layers.